Control apparatus for an internal combustion engine and method for controlling the same

ABSTRACT

In a control apparatus for an internal combustion engine having a valve timing control portion controlling the timing opening and closing of an intake valve disposed in an intake port communicating with a cylinder of the internal combustion engine, when the internal combustion engine is being started from a cold condition, multi-stroke operation is set, in which one combustion cycle of the internal combustion engine includes two or more intake and compression strokes, formed by a first intake stroke and the first compression stroke and a second intake stroke and compression stroke, followed by a combustion stroke and an exhaust stroke. The valve timing control portion controls a lift of the intake valve during the first intake stroke and the first compression stroke to a low lift amount, which is smaller than the normal lift amount required for intake of a requested intake air amount, and controls the lift of the intake valve in a second intake and a second compression stroke to the normal lift amount.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control apparatus for an internalcombustion engine and a method for controlling an internal combustionengine. More specifically, it relates to a control apparatus for aninternal combustion engine and a method for controlling an internalcombustion engine capable of controlling the timing of the opening andclosing and the lift amounts of individual intake valves of an internalcombustion engine.

2. Description of the Related Art

The Japanese Patent Application Publication No. JP-A-10-252511 disclosesa system that controls the opening and closing of the intake valve andexhaust valve by a valve driving mechanism capable of variably adjustingthe timing of the opening and closing of intake and exhaust valvesdisposed in each cylinder of an internal combustion engine. In thissystem, during normal operation, in which combustion in the internalcombustion engine is stable, the internal combustion engine is operatedby a four-stroke combustion cycle comprising an intake stroke, acompression stroke, an expansion stroke, and an exhaust stroke. In doingthis, control is performed to open and close the intake valve at aprescribed timing in the intake stroke, and control is performed to openand close the exhaust valve at a prescribed timing in the exhauststroke.

However, in the case, for example, of a cold start of the internalcombustion engine, in which there is a tendency for incompletecombustion to occur, there are cases in which the fuel is not completelycombusted and non-combusted fuel remains that is exhausted from thecylinder in the exhaust stroke. Therefore, in the above-noted system, inthe case in which it is determined that incomplete combustion isoccurring in the internal combustion engine, the following control isperformed. Specifically, the opening of the intake valve and the exhaustvalve is stopped for a prescribed period of time during operation of theinternal combustion engine. As a result, during the time that the valvesare stopped, both the intake and the exhaust valves are closed, and theup and down motion of the piston repeatedly performs the compressionstroke and the expansion stroke only. In this condition, ignition isdone each time between the compression stroke and the expansion stroke.

During this repetition of the compression and explosion/expansionstrokes in this manner, non-combusted fuel in the cylinder is completelycombusted. Subsequently, when complete combustion of the non-combustedfuel is verified, normal valve opening and closing operation is againpermitted and normal operation resumes. By doing this, the systemenables complete fuel combustion and suppression of exhausting ofnon-combusted fuel in an operating condition in which there is atendency for incomplete combustion to occur.

During the time when an internal combustion engine is being started fromthe cold condition of the internal combustion engine, in order tostabilize combustion and to improve starting characteristics, the fuelinjection amount is increased by performing control to achieve afuel-rich condition. From the standpoint of improving fuel economy andexhaust emissions, however, it is desirable to further expand the leanlimit of the internal combustion engine even during the cold start. Withregard to this point, when incomplete combustion occurs, the above-notedsystem performs repeated compression and expansion strokes with both theintake valve and the exhaust valve closed, performing ignition each timeto complete combust the non-combusted fuel. That is, according to thesystem, by repeating the compression and explosion/expansion strokes,fuel that fills the closed cylinder is completely combusted, and is notrelated to control of the air-fuel ratio toward the lean side duringoperation when the internal combustion engine is cold started, nor is itrelated to extending the lean limit.

SUMMARY OF THE INVENTION

The present invention has an object to provide a control apparatus foran internal combustion engine and a method for controlling an internalcombustion engine improved to achieve an extension of the lean limit,even when the internal combustion engine is being started from the coldstart.

A first aspect of the present invention is a control apparatus for aninternal combustion engine having a variable valve driving means forchanging the timing of the opening and closing and lift amount of anintake valve disposed in an intake port communicating with a cylinder ofthe internal combustion engine; a valve timing control means forcontrolling the timing of the opening and closing and lift amount of theintake valve by the variable valve driving means; a cold startdetermining means for determining whether the internal combustion engineis being started from a cold start; and a multi-stroke operation settingmeans for setting, in which one combustion cycle of the internalcombustion engine includes two or more intake and compression strokes,when the cold start determining means determines that the internalcombustion is being started from the cold start, wherein themulti-stroke operation is formed by a first intake stroke and a firstcompression stroke and a second intake stroke and a second compressionstroke, followed by a combustion stroke and an exhaust stroke. In thisaspect, the valve timing control means controls a lift of the intakevalve during the first intake stroke and the first compression stroke toa low lift amount, which is smaller than the normal lift amount requiredfor intake of a requested intake air amount, and controls the lift ofthe intake valve in a second intake stroke and a second compressionstroke to the normal lift amount.

According to the first aspect, by performing two or more intake andcompression strokes, and making the lift amount in the first intakestroke small, it is possible to raise the intake temperature when anintake air flows into a combustion chamber. Even when the temperature ofthe internal combustion engine is low, therefore, as during coldstarting, it is possible to more quickly raise the temperature in thecombustion chambers and stabilize combustion.

In a second aspect, the low lift amount may be the lift amount at whichthe pumping loss during the first intake stroke and the firstcompression stroke is maximum.

According to the second aspect, when intake is done in the first intakestroke and the first compression stroke, it is possible to moreeffectively raise the temperature of the intake gas, enabling not onlyan improvement in combustion characteristics, but also earlier warm-upof the internal combustion engine.

A third embodiment is the control apparatus of either the first orsecond aspect, which may further have an ignition timing control meansfor controlling ignition timing by a spark plug disposed in thecylinder, wherein the ignition timing control means prohibits ignitionduring the first intake stroke and the first compression stroke.

According to the third aspect, in addition to effectively raising thetemperature of the intake gas during the first intake stroke and thefirst compression strokes, it is possible to perform intake inaccordance with a requested intake air amount in the second intake andthe second compression stroke, enabling generation of a torque asrequired for the requested load.

A fourth aspect is a-control apparatus of any one of the first to thirdaspects, wherein the multi-stroke operation setting means may perform,during one combustion cycle, a plurality of repetitions of the firstintake stroke and the first compression stroke, followed by performingthe second intake stroke and the second compression stroke.

According to the fourth aspect, the intake temperature is more reliablyraised and it is possible to warm up the internal combustion engine atan earlier stage.

A fifth aspect is the control apparatus according to any of the first tofourth aspects, which may further have a multi-stroke operationtermination determining means for determining whether multi-strokeoperation is to be terminated; and a four-stroke operation setting meansfor setting one combustion cycle of the combustion of the internalcombustion engine to four-stroke operation comprising an intake stroke,a compression stroke, an expansion stroke, and an exhaust stroke, if themulti-stroke operation judges that multi-stroke operation is to beterminated.

The above-noted multi-stroke operation is advantageous in improving thecombustion characteristics when the internal combustion engine is cold.However, because the lift amount of the intake valve becomes small inthe first intake stroke and the first compression stroke, so that theintake resistance becomes large, the torque loss is large. Therefore,after combustion stabilizes, a switch may be made to normal four-strokeoperation. With regard to this point, according to the sixth throughninth aspects as noted below, it is possible to reliably determine thetiming of this switching and, when it is determined that themulti-stroke operation is to be terminated, it is possible to switch tofour-stroke operation, in which the normal intake stroke, compressionstroke, expansion stroke, and exhaust stroke are performed.

A sixth aspect is the control apparatus of the fifth aspect, which mayfurther have a temperature detecting means for detecting a temperaturein the cylinder, wherein the multi-stroke operation terminationdetermining means determines that multi-stroke operation is to beterminated if a temperature in the cylinder reaches a threshold cylindertemperature.

A seventh aspect is the control apparatus of the fifth aspect, which mayfurther comprise a water temperature detection means for detecting thetemperature of the coolant of the internal combustion engine, whereinthe multi-stroke operation termination determining means determines thatmulti-stroke operation is to be terminated if the coolant temperaturereaches a threshold coolant temperature.

An eighth aspect is the control apparatus of the fifth aspect, which mayfurther have a requested load calculation means for calculating arequested load on the internal combustion engine, wherein themulti-stroke operation termination determining means determines thatmulti-stroke operation is to be terminated if the calculated requestedload reaches or exceeds a threshold engine load.

A ninth aspect is the control apparatus of the fifth aspect, which mayfurther have a cylinder temperature predicting means for predicting,before starting the first intake stroke and the first compression strokein one combustion cycle, a temperature in a cylinder after performingthe second intake stroke and the second compression stroke, wherein themulti-stroke operation termination determining means determines thatmulti-stroke operation is to be terminated if the predicted temperaturein the cylinder reaches a threshold predicted cylinder temperature.

In the case of the multi-stroke operation noted above, if intake andcompression strokes are repeated, the intake gas temperature will raiseexcessively. When the intake gas temperature rises excessively, it isalso possible to envision this as a cause of abnormal combustion. Withregard to this point, according to the ninth aspect, the temperature inthe cylinder is predicted, and it is determined whether to switchingfrom multi-stroke operation to six-stroke operation based on thepredicted temperature. Therefore, even in the case of performingmulti-stroke operation, it is possible to more reliably prevent theintake gas from reaching an excessively high temperature.

Also, in the case in which the fuel of the internal combustion engineincludes an alcohol, because the volatility of the fuel will differdepending on the concentration of the alcohol fuel in the fuel, thecombustion characteristics will also change. That is, even for the sameoperating condition, the time from starting to reach stabilizedcombustion will differ.

A tenth aspect is the control apparatus of any one of the sixth to ninthaspects, wherein the internal combustion engine may use a fuel includingalcohol as a fuel, and the control apparatus may set any one of thethreshold cylinder temperature, the threshold coolant temperature, thethreshold engine load, and the threshold predicted engine temperature inaccordance with the concentration of alcohol fuel in the fuel.

According to the tenth aspect, the determination value of the thresholdcylinder temperature, the threshold coolant temperature, the thresholdengine load, or the threshold predicted engine temperature used as areference in determining whether to switch from multi-stroke operationto four-stroke operation in accordance with the concentration of alcoholfuel in the fuel. It is therefore possible to perform the switchingdetermination reliably, in accordance with the fuel that is used.

An eleventh aspect is the control apparatus of any one of the first totenth aspects, wherein the internal combustion engine may a firstcylinder group and a second cylinder group, and wherein the controlapparatus may operate only cylinders belonging to the first cylindergroup and may include a reduced-cylinder operation setting means forsetting cylinders belonging to the second cylinder group to reducedcylinder operation, in which the cylinders are stopped, and anall-cylinder operation setting means for setting all cylinders belongingto the first cylinder group and cylinders belonging to the secondcylinder group to all-cylinder operation, in which all cylinders areoperated, wherein the cold start determination means determines whetherrestoration of operation of the cylinders belonging to the second groupof cylinders is a cold start when an engine transition is made fromreduced-cylinder operation to all-cylinder operation, and themulti-stroke operation setting means sets the operation of cylindersbelonging to the second cylinder group to multi-stroke operation whenthe cold start determining means determines that restoration ofoperation of the cylinders belonging to the second group of cylinders isthe cold start.

According to the eleventh aspect, when return is being made fromso-called reduced-cylinder operation to all-cylinder operation, even inthe case in which the cylinders that had be stopped duringreduced-cylinder operation are to be restored to operation when cold, itis possible to apply the above-noted multi-stroke operation. It istherefore possible to more quickly stabilize the combustion in cylindersthat had been stopped, and possible to more quickly return fromreduced-cylinder operation to all-cylinder operation.

A twelfth aspect is the control apparatus of any one of the first toeleventh aspects, wherein the variable valve driving means may have anintake cam driving the opening and closing of the intake valve and anelectrical motor rotationally driving the intake cam, wherein the valvetiming control means may control the valve timing by controlling therotational drive of the intake cam using the electrical motor.

According to the twelfth aspect, because it is possible to control thevalve timing of the intake valve using an electrical motor, it ispossible to reliably control the intake valve to a set lift amount, andpossible to reliably achieve control of the lift amount in theabove-noted six-stroke operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features, and advantages of theinvention will become apparent from the following description ofpreferred embodiments with reference to the accompanying drawings,wherein like numerals are used to represent like elements, and wherein:

FIG. 1 is a schematic view describing the configuration of a systemaccording to the first embodiment of the present invention;

FIG. 2 is a drawing describing the relationship between the lift amountof an intake valve and the pumping loss;

FIG. 3A and FIG. 3B are drawings describing the opening and closingtiming and the lift amounts of the intake and exhaust valves;

FIG. 4 is a flowchart describing a control routine executed by thesystem in the first embodiment of the present invention;

FIG. 5 is a graph describing the relationship between the coolanttemperature and the threshold engine load in a second embodiment of thepresent invention;

FIG. 6 is a flowchart describing a control routine executed by thesystem in the second embodiment of the present invention;

FIG. 7 is a flowchart describing a control routine executed by thesystem in the third embodiment of the present invention;

FIG. 8 is a graph describing the relationship between the alcoholconcentration in the fuel and the threshold coolant temperature in afourth embodiment of the present invention;

FIG. 9 is a flowchart describing a control routine executed by thesystem in the fourth embodiment of the present invention; and

FIG. 10 is a flowchart describing a control routine executed by thesystem in a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention are described in detail below, withreferences made to the accompanying drawings. In the drawings, elementsthat are the same or corresponding elements to elements earlierdescribed are assigned the same reference numerals and the descriptionsthereof are either simplified or omitted.

FIG. 1 is a schematic view showing the configuration of the firstembodiment of the present invention. The system shown in FIG. 1 has aninternal combustion engine 10. The internal combustion engine 10 has acylinder 12. Although only the cross-section of only one cylinder 12 isshown in FIG. 1, the internal combustion engine 10 actually has aplurality of cylinders 12. A piston 14 is disposed within the cylinder12. The piston 14 is connected to a crankshaft 18 via a connecting rod16. A rotational speed sensor 20 that generates an output responsive tothe rotational speed of the crankshaft is disposed in the vicinity ofthe crankshaft 18. A coolant temperature sensor 22 that detects thetemperature of the coolant for the internal combustion engine isprovided in the internal combustion engine. A combustion chamber 24 isprovided at the top of the piston 14. A temperature sensor 26(temperature detecting means) that generates an output responsive to thetemperature within the combustion chamber 24 is disposed in thecombustion chamber 24. A spark plug 28 is inserted with the head thereofexposed into the combustion chamber 24.

The internal combustion engine 10 has an intake port 30 and an exhaustport 32 that communicate with the combustion chamber 24. An injector 34is built into the intake port 30. The intake port 30 is connected to anintake passage 36. An air flow meter 38 is disposed in the intakepassage 36.

The intake port 30 of each cylinder 12 of the internal combustion engine10 has an intake valve 40 that opens and closes the intake port 30. Anintake valve shaft 42 is fixed to the intake valve 40. A valve lifter 44is mounted to the top end of the intake valve shaft 42. The impellingforce of a valve spring 46 acts on the intake valve shaft 42, and theintake valve 40 is impelled in the valve-closing direction by theimpelling force. An intake cam 50 is disposed above the valve lifter 44.The intake cams 50 of each cylinder 12 are connected two each to one andthe same camshaft (not shown), and are linked to a variable valve timingmechanism 52 via the camshaft and the like. A cam position sensor 54 ismounted in the vicinity of the camshaft of the intake cams. The camposition sensor 54 generates an output responsive to the rotationalangle and the rotational speed of the intake cam 50.

The internal combustion engine 10 has, on the exhaust ports 32 of eachcylinder 12, an exhaust valve 60 that opens and closes the exhaust port32. The exhaust valve 60 has the same configuration as the intake valve40. That is, the exhaust valve 60 has an exhaust valve shaft 62 fixed tothe exhaust valve 60, a valve lifter 64 mounted at the top of theexhaust valve shaft 62, and a valve spring 66 mounted so as to impel theexhaust valve shaft 62 in the valve-closing direction. An exhaust cam 70is disposed at the top of the valve lifter 64. The exhaust cams 70 ofeach cylinder 12 are connected two each to one and the same camshaft(not shown), and are linked to a variable valve timing mechanism 72 viathe camshaft and the like. A cam position sensor 74 is mounted in thevicinity of the camshaft of the exhaust cams 70. The cam position sensor74 generates an output responsive to the rotational angle and therotational speed of the exhaust cam 70.

The variable valve timing mechanism 52 for the intake valve 40 sideutilizes a motor to control the rotational speed and rocking of thecamshaft to control the rotation an the rocking of the intake cam 50. Asa result, the phase, operating angle, and lift amount of the intakevalve 40 can be varied independently for each cylinder 12. The variablevalve timing mechanism 72 for the exhaust valve 70 utilizes a motor orthe like to control the rotation and rocking of the camshaft to controlthe rotation and rocking of the exhaust cam 70. As a result, the phase,operating angle, and lift amount of the exhaust valve 70 can be variedindependently for each cylinder 12.

By varying the phase of the intake and exhaust valves 40, 60, it ispossible to change the timing of the opening and the closing of theintake and exhaust valves 40, 60. By varying the operating angle, it ispossible to change the period of time of opening of the intake andexhaust valves 40, 60. By varying the lift amount, it is possible tochange the size of the passage formed between the valves and the intakeand output ports 30, 32 when the intake and exhaust valves 40, 60 areopen. This type of control can be performed for each intake valve 40 andexhaust valve 60 of each individual cylinder 12. Because the mechanismto control the rotating and rocking of the camshaft so as to control thephase, operating angle, and lift amount of the intake valves 40 or theexhaust valve 60 is not particularly novel, it will not be describedhere in detail.

The internal combustion engine 10 has an ECU (electronic control unit)80 as a control apparatus for the internal combustion engine. The ECU 80acquires information required for control of the internal combustionengine 10 from such sensors as the rotational speed sensor 20, thecoolant temperature sensor 22, the temperature sensor 26, the air flowmeter 38, and the cam position sensors 54, 74, and controls the sparkplug 28, the injector 34, an the variable valve timing mechanisms 52,72, based on the acquired information.

In cases, for example, in which the internal combustion engine 10 iscold started, in which the internal combustion engine 10 has not yetbeen warmed up, the temperatures of various parts of the internalcombustion engine 10 are low. For this reason, the intake airtemperature when the engine is cold started is low. It is thereforedifficult for fuel to be atomized, and the condition occurs in whichthere is low mixture of air and fuel, making it difficult to achievestable combustion. For this reason, control is generally performed toincrease the amount of fuel injection amount at the cold start. If thiscontrol is performed, however, the air-fuel ratio during the cold startbecomes rich, and the amount of non-combusted fuel that is exhaustedincreases. In order to extend the lean limit and improve the fueleconomy and exhaust emissions characteristics, therefore, it isdesirable that the control to increase the fuel injection amount duringa cold start be done over a short period of time, or be avoided. Forthis reason, it is desirable in cold starting that, at an earlier stageafter the starting of the internal combustion engine 10 the intake airtemperature is increased, atomization is promoted, and combustion isstabilized.

If the intake resistance in the intake stroke of the internal combustionengine 10 increases, heat of friction occurs when the intake gas istaken into the cylinder 12. Therefore, by making the intake resistancein the intake stroke, that is, the pumping loss, large it is possible toraise the temperature of the gas taken into the cylinder 12 byincreasing heat of friction.

FIG. 2 is a drawing describing the relationship between the lift amountof the intake valve 40 and the pumping loss. In FIG. 2, the horizontalaxis represents the lift amount of the intake valve 40, and the verticalaxis represents the pumping loss. The solid line (i) in FIG. 2 shows thecase in which the internal combustion engine 10 is in a lower rotationalspeed region than shown by the solid line (ii). As shown in FIG. 2, asthe lift of the intake valve 40 increased from the condition in whichthe lift amount is zero, the pumping loss also increases, and becomesmaximum at some lift amount (low lift amount). The pumping lossgradually decreases as the lift amount becomes larger than the low liftamount. The low lift amount at which the pumping loss is maximum isdifferent depending upon the engine rotational speed and, as shown bythe solid lines (i) and (ii) of FIG. 2, as the engine rotational speedincreases, the low lift amount tends to increase.

The heat of friction generated when the intake gas flows in cylinder ishigher, the greater is the pumping loss in the intake stroke. Therefore,by setting the lift amount to the low lift amount and making the pumpingloss maximum, it is possible to more quickly raise the temperature ofthe intake gas when making a cold start.

In contrast, the lift amount of the intake valve 40 is set to the liftamount (normal lift amount) required to reliable intake the requestedair into the cylinder 12. With regard to this point, even when theinternal combustion engine is cold started, in order to generate thetorque required for starting, it is necessary to intake the requestedamount of air, and necessary to perform intake by controlling the liftamount of the intake valve 40 to the normal lift amount. Therefore, ifthe compression stroke and the explosion/combustion stroke are performedafter completing the intake stroke with the control remaining at the lowlift amount in order to raise the temperature as noted above, it can bepresumed that the amount of intake gas filling the cylinder 12 isinsufficient, so that the combustion is actually decreased.

The system of the first embodiment, in order to simultaneously achieve arise of the intake gas temperature and a sufficient gas intake amount inperforming a cold start, after repeating the intake stroke and thecompression stroke two times in one combustion cycle of the internalcombustion engine 10, executes a combustion stroke and an exhauststroke. FIG. 3A and FIG. 3B describe the opening and closing timing andlift amounts of the intake and exhaust valves in the internal combustionengine 10, FIG. 3A showing operation at the time of cold starting andFIG. 3B showing the operation after combustion stabilizes.

As shown in FIG. 3A, when the intake temperature is low during a coldstart, one combustion cycle is formed by the six strokes of the firstintake stroke, a first compression stroke, a second intake stroke, asecond compression stroke, an expansion stroke, and an exhaust stroke,with one ignition occurring with a prescribed timing in the secondcompression stroke. In this embodiment, this operating condition of theinternal combustion engine 10 will be referred to as “six-strokeoperation.” During six-stroke operation, in the initial intake stroke(first intake stroke), in order to raise the intake temperature with thepumping loss at a maximum, the lift amount of the intake valve 40 iscontrolled to be the low lift amount. The relationship between the lowlift amount and the pumping loss differs, depending upon the timing ofthe opening and closing and the operating angle of the intake valve.Therefore, the low lift amount that is set at this point is the liftamount that maximizes the pumping loss under a condition in which theopening and closing timing and operating angle are set to appropriatetiming and angle in relationship to other operating conditions. Theintake temperature rises because of the heat of friction generatedduring the first intake stroke. Although the temperature rise willdiffer depending upon the temperature of the gas that is taken in andthe engine rotational speed at that time, it is, for example,approximately 50° C. to 60° C. After performing intake in the conditionof this lift amount, the intake valve 40 is closed, and the firstcompression stroke is entered.

After the above, the second intake stroke is entered without performingignition. When this occurs, the lift amount of the intake valve 40 iscontrolled to the normal lift amount required to intake the requestedair amount. The normal lift amount also differs depending upon thetiming of the opening and closing and the operating angle of the intakevalve 40. Therefore, the normal lift amount is set to a lift amount tointake the requested amount of air in the case in which the lift amount,the opening and closing timing, and the operating angle are properlyset. With the valve timing set in this manner, the piston 14 is loweredin the second intake stroke and intake is performed. By doing this, therequired amount of intake air can be acquired. The piston 14 begins torise and the second compression stroke starts, and in the secondcompression stroke ignition is performed at the optimum timing. Afterthat, the expansion stroke and the exhaust stroke are performed.

During the six-stroke operation, the exhaust valve 60 is closed during aperiod from the first intake stroke to the second compression stroke andduring the compression stroke in the same manner as a period from thenormal intake stroke to the expansion stroke. That is, during thesix-stroke operation control is performed so that the exhaust valve 60is first opened and then closed at an appropriate time in the region ofthe start of the exhaust stroke.

The two intake and compression strokes causes the temperature of theintake gas to raise, and enables intake of the required amount of intakegas. Therefore, it is possible to acquire the air required forcombustion while promoting the mixing of fuel and air, and possible toimprove the combustion condition during the cold start. Also, by atemperature rise of the intake gas, it is possible to more quicklywarm-up the internal combustion engine and stabilize combustion. Becausethe raising of the intake gas temperature at the time of cold startstabilizes combustion, it is possible to suppress an increase in thefuel injection amount, and extend the lean limit.

A map establishing the relationship between the low lift amount andengine rotational speed at which the pumping loss is maximum and thetiming of the opening and closing of the intake valve 40, and a mapestablishing the relationship between the lift amount, the requested airintake amount and timing of the opening and closing of the intake valve40 are stored in the ECU 80. The low lift amount and normal lift amountin the case of six-stroke operation are established in accordance withthese maps, and the ECU 80 performs control of the intake valve 40, viathe variable valve timing mechanism 52, in accordance with the set lowlift amount and normal lift amount.

After the internal combustion engine 10 is warmed up and combustion hasstabilized, a transition is made to operation (four-stroke operation) inwhich one combustion cycle is formed by the normal four strokes. Thatis, the intake stroke is performed so that the lift amount of the intakevalve 40 is controlled to the normal lift amount in response to therequested amount of intake air. After that, the intake valve 40 isclosed and, after performing the compression stroke, ignition isperformed at an appropriate timing, immediately after which theexpansion stroke and exhaust stroke are performed. The exhaust valve 60,similar to the case of normal control, is first opened and then closedat an appropriate time in the exhaust stroke of the four strokes toperform exhausting.

The ECU 80 stores a map establishing the relationship between the normallift amount, the amount of requested air intake, and the timing of theopening and closing and the operating angle of the intake valve 40. Inthe case of performing four-stroke operation, the normal lift amount iscalculated using the map and, in response to the calculated normal liftamount, the ECU 80 controls the intake valve 40 via the variable valvetiming mechanism 52.

The six-stroke operation is effective in the case in which, for example,during cold starting, when raising the intake temperature has priority.However, in the condition in which control is performed to the low liftamount at which the pumping loss is maximum, because two intakes areperformed the torque loss increases. If the internal combustion engine10 has been warmed up and the combustion has been stabilized, a switchis made immediately to four-stroke operation. For this reason, it isdetermined in the system of the first embodiment that combustion hasstabilized, a transition is made from six-stroke operation tofour-stroke operation.

Specifically, when the temperature in the combustion chamber 24 hasrisen sufficiently, it can be determined that combustion in the internalcombustion engine 10 has stabilized. Therefore, the temperature in thecombustion chamber 24 is detected from the output of the temperaturesensor 26 mounted in the combustion chamber 24, and if the detectedtemperature is sufficiently high, it is determined that combustion hasstabilized. The ECU 80 has stored a threshold cylinder temperature thatis the minimum temperature in the combustion chamber 24 to determinethat the internal combustion engine 10 has warmed up and the combustionhas stabilized. If the detected temperature has reached at least thethreshold cylinder temperature, the ECU 80 determines that combustion inthe internal combustion engine 10 has stabilized and switches fromsix-stroke operation to four-stroke operation.

FIG. 4 is a flowchart describing a control routine executed by the ECU80 in the first embodiment of the present invention. The flowchart shownin FIG. 4 is a routine that executed each time the internal combustionengine 10 is started. In the flowchart shown in FIG. 4, the temperatureof the coolant in the internal combustion engine 10 is first detected(step S100). The coolant temperature is determined based on the outputof the coolant temperature sensor 22. Next, it is determined whether ornot cold start of the internal combustion engine 10 has been requested(step S102). Whether or not cold start has been requested is determined,for example, based on whether or not the coolant temperature determinedin step S100 is below a prescribed range when starting of the internalcombustion engine 10 is requested.

If it is determined at step S102 that the cold start has been requested,required information regarding the current operating condition isdetected (step S104). For example, information such as the enginerotational speed, the accelerator operating amount, and the temperaturein the combustion chamber 24 is detected in accordance with the outputfrom various sensors. Next, the requested intake air amount iscalculated (step S106). The requested intake air amount is calculated inaccordance with requested load determined based on the output of anaccelerator operating sensor.

Next, it is determined whether or not the temperature T24 in thecombustion chamber 24 is greater than or equal to the threshold cylindertemperature T0 (step S108). If at step S108 the temperature T24 in thecombustion chamber 24 is greater than or equal to the threshold cylindertemperature T0, it is determined that the internal combustion engine 10has not warmed up and that combustion has not stabilized, resulting inexecution of six-stroke operation (step 110).

Specifically, using a map stored in the ECU 80, the low lift amount atwhich the pumping loss is maximum for the current engine rotationalspeed is determined, and the lift amount of the intake valve 40 for thefirst intake stroke is determined. Using a map stored in the ECU 80, thenormal lift amount for the second intake stroke is determined inaccordance with the requested intake air amount determined in step S106.In accordance with the current operating condition detected in stepS104, the operating angle and phase of the intake valve 40 at the timeof engine starting are determined. In accordance with the valve timing,such as the calculated lift amount and the like, control of the intakevalve 40 is performed by the variable valve timing mechanism 52. In thiscondition, the first intake stroke, the first compression stroke, thesecond intake stroke, and the second compression stroke are performed,after which the compression stroke and exhaust stroke are performed.Control is performed so that ignition is done at an appropriate timeduring the second compression stroke. During this period of time,control is performed to close the exhaust valve 60 from the first intakestroke to the second compression stroke and during the expansion stroke,and to open the exhaust valve 60 at the normal valve timing in theexhaust stroke to perform exhausting.

Return is made again to step S104, at which information regarding thecurrent operating condition is detected after the requested amount ofintake air is calculated (steps S104 and S106), at step S108 thetemperature T24 in the combustion chamber 24 is compared with thethreshold cylinder temperature T0. If it is not determined that thetemperature T24 is greater than or equal to the threshold cylindertemperature T0 at step S108, six-stroke operation is performed (stepS110), and the processing of steps S104 to S108 is performed. That is,six-stroke operation (step S110) and the processing of steps S104 toS108 are repeated until it is determined that the temperature T24 in acombustion chamber temperature reaches the threshold cylindertemperature T0 at step S108.

If, however, it is not determined at step S102 that a cold start hadbeen requested, and if it is determined at step S108 that thetemperature T24 in the combustion chamber 24 is greater than or equal tothe threshold cylinder temperature T0, it is determined that thetemperature T24 in the combustion chamber 24 has reached the temperatureT0 at which the internal combustion engine is assumed to have alreadywarmed up. Therefore, normal four-stroke operation is executed (stepS112). Specifically, using a map stored in the ECU 80, the normal liftamount of the intake valve 40 is set in accordance with the requestedamount of intake air. The opening and closing timing and operatingangles of the exhaust valve 60 and the intake valve 40 are set inaccordance with the current condition of the internal combustion engine.In this condition, the normal intake stroke, compression stroke,expansion stroke, and exhaust stroke are performed, and control is doneto perform ignition between the compression stroke and the expansionstroke. Next, the processing is ended.

As described above, according to the first embodiment when performingcold starting, control is performed so that after performing the firstintake stroke and the first compression stroke at the low lift amount atwhich the pumping loss is maximum, the second intake stroke and thesecond compression stroke are performed, after which the expansionstroke and the exhaust stroke are performed. By doing this, it ispossible to raise the temperature of the intake air from the conditionin which the intake temperature is low, such as when doing coldstarting, thereby enabling both an improvement in the combustioncondition, early warm-up of the internal combustion engine, andstabilization of combustion.

In the first embodiment, the temperature T24 in the combustion chamber24 is directly detected and, based on whether this temperature T24 hasreached the threshold temperature T0, it is determined whether to switchbetween six-stroke operation and four-stroke operation. The presentinvention, however, is not limited to performing the determination ofswitching between six-stroke operation and four-stroke operation in thismanner. This determination can be made if it is possible to determinewith some accuracy the stabilization of combustion when cold starting isdone. The determination of whether or not to switch, therefore, can bemade, for example, by detecting the temperature of the coolant for theinternal combustion engine 10 and basing the judgment on whether or notthe coolant temperature is higher than or equal to the threshold coolanttemperature at which the internal combustion engine 10 is assumed tohave warmed up. The threshold coolant temperature can be set based on atemperature by experimentally determining a value which isexperimentally determined and whish indicates that the internalcombustion engine 10 has warmed up and, based on that value, inconsideration of what extent of warm-up the six-stroke operation is tobe continued.

The first embodiment of the present invention is described for the casein which it is determined that a cold start has been requested. Thepresent invention is not, however, limited in this manner, and mayperform six-stroke operation in other cases in which it is effective togive priority to raising the intake temperature. Therefore, for example,six-stroke operation may be started even in cases in which it isdetermined that the internal combustion engine 10 is not warmed up.Also, six-stroke operation may also be performed only in a case, of aso-called fast idling condition, in which the internal combustion engineis operating at a rotational speed higher than a normal idling speed,such as during catalyst warm-up or during a cold start.

The low lift amount in the first intake stroke of six-stroke operationwas described for the case of a lift amount at which the pumping loss ismaximum. In the present invention, however, the low lift amount is notlimited in this manner, and may be set as a small lift amount at whichthe pumping loss is larger than at a lift amount that is normally set inaccordance with a requested amount of intake air. This is a reason why,even if two intake strokes are performed at a lift amount that is thesame as the normal lift amount, it is possible to raise the intaketemperature slightly.

The first embodiment was described for the case in which the lift amountin the second intake stroke during six-stroke operation and the liftamount in four-stroke operation are set in accordance with the requestedamount of intake air, and valve timing control including this liftamount is performed to control the intake air amount. The presentinvention is not limited in this manner, however, and an electronicallycontrolled throttle valve may be disposed in the intake passage 36 andthe air intake amount may be controlled by the degree of opening of thethrottle valve. In this case, the lift of the intake valve in the firstintake stroke can be controlled to the low lift amount, and the normallift amount in the second intake stroke and in four-stroke operation canbe set to the maximum lift amount for the case in which the intake cam50 is rotated one time, instead of the lift amount set in accordancewith the requested amount of intake air.

Also, the first embodiment is described for case in which, during thecold start of the internal combustion engine, the first combustion cycleis performed with six-stroke operation, which includes a first intakestroke, a first compression stroke, a second intake stroke, a secondcompression stroke, an expansion stroke, and an exhaust stroke. However,the present invention is not limited in this manner, and multi-strokeoperation may be performed that includes a plurality of repetitions of afirst intake stroke and a first compression stroke, followed byperforming of a second intake stroke, a second compression stroke, anexpansion stroke, and an exhaust stroke. In this case, pumping lossduring the first intake stroke increases, thereby enabling ofeffectively raising the intake temperature during one combustion cycle.

The first embodiment was described for the case in which the means forchanging the valve timing of the intake valve 40 is that of connectingtwo intake cams 50 each to one and the same camshaft, the rotation androcking of the camshaft being controlled by the variable valve timingmechanism 52, and value timing including the phase, lift amount, andoperating angles of the intake valves 40 being controlled independentlyfor each cylinder 12. The present invention, however, is not limited tothis method of controlling the intake valve 40. In the presentinvention, the means for changing the valve timing of the intake valve40 may be a different configuration that is capable of opening andclosing the valve at least two times, in the intake strokes during onecombustion cycle, and also changing the lift of the intake valve.Specifically, for example, by using an electromagnetically driven valve,the lift and opening and closing timing of the intake valve 40 may beindependently controlled for each intake valve 40. In the same manner,the means for changing the valve timing of the exhaust valve 60 is notlimited to the means described with regard to the first embodiment, andmay be a different configuration that is capable of controlling thetiming of the one opening and closing at appropriate times within theexhaust stroke, in accordance with the condition in six-stroke operation(or multi-stroke operation).

Although the first embodiment was described for the case in which theinternal combustion engine 10 is a gasoline engine, there is nolimitation in this manner, and the internal combustion engine 10 mayalso be, for example, a diesel engine. Although the example given isthat of fuel injection by port injection, the engine may be an internalcombustion engine that uses cylinder injection.

For example, in the first embodiment by executing step S102 the “coldstarting determining means” may implemented, by executing step S110 the“multi-stroke operation setting means,” the “valve timing controlmeans,” and the “ignition timing control means” may be implemented, byexecuting step S108 the “multi-stroke operation termination determiningmeans” may be implemented, by executing step S110 the “four-strokeoperation setting means” is implemented, and by executing step S104 the“temperature detection means” and “coolant temperature detection means”are implemented.

The second embodiment of the present invention is described below, withreference to FIG. 5 and FIG. 6. The description of the second embodimentwill focus on the only the characteristic parts of the secondembodiment, and the descriptions of parts that are the same as the firstembodiment will be either simplified or omitted. The system in thesecond embodiment has the same type of configuration as the system ofthe first embodiment. In the system of the second embodiment, with theexception of the method for determining timing of switching fromsix-stroke operation to four-stroke operation being different from themethod in the first embodiment, control is performed in the same manneras in the first embodiment.

Specifically, in the system of the second embodiment, the determinationof the switching from six-stroke operation to four-stroke operation ismade in accordance with an engine load. FIG. 5 is a graph describing therelationship between the coolant temperature and the threshold engineload to determine whether to switch from six-stroke operation tofour-stroke operation in the second embodiment. In FIG. 5, thehorizontal axis represents the coolant temperature and the vertical axisrepresents the threshold engine load. As noted above, in the six-strokeoperation, the intake stroke are performed two times and, of the twointake strokes, intake is performed in the first intake stroke with thelift amount set to the lift amount at which the pumping loss is maximum.For this reason, compared with the case of normal four-stroke operation,the generated torque is small. Therefore, in the case in which the loadbecomes large, it is difficult to generate a torque in accordance withthat load with six-stroke operation. Therefore, regardless of whetherthe internal combustion engine 10 has warmed up, in the case in whichthe requested load is above a given load, in order to generate a torquein accordance with the requested load, switching is done from thesix-stroke operation to the four-stroke operation. That is, the rise ofthe requested load of the internal combustion engine 10 above the solidline (i) (threshold engine load (i)) in FIG. 5 is a first condition forswitching from six-stroke operation to four-stroke operation.

In six-stroke operation, an increase in the intake temperature makes iteasier to burn the fuel. If the engine load becomes large under thiscondition, abnormal combustion tends to cause knocking. Also, if suchabnormal combustion occurs, it can be assumed that the warm-up of theinternal combustion engine 10 has progressed to some extent. In thesecond embodiment, therefore, in order to give priority to suppressionof knocking, six-stroke operation is only permitted only when the engineload is within a range that does not cause knocking. The limit value ofrequested load set in consideration of occurrence of knocking, asindicated by the solid line (ii) in FIG. 5, is smaller than when thetemperature of the coolant for the internal combustion engine 10 ishigh, and becomes higher when the coolant temperature is low. Inconsideration of suppression of knocking, the rise above the solid line(ii) in FIG. 5 (threshold engine load (ii)) is a second condition forswitching from six-stroke operation to four-stroke operation.

From the above, in the second embodiment six-stroke operation isexecuted at the cold start, and a transition is made to four-strokeoperation if either of the following first and second conditions issatisfied. The first condition is (requested load)≧(threshold engineload (i)) and the second condition is (requested load)≧(threshold engineload (ii)).

That is, when the coolant temperature and the requested load are in theregion below the thick line (I), six-stroke operation is executed andcontinued, and the value of the solid line (I) is a threshold engineload for switching from six-stroke operation to four-stroke operation.The threshold engine load is the value that is the smaller of thethreshold engine load (i) and the threshold engine load (ii) at thecoolant temperature at that time. The ECU 80 has stored map establishingthe relationship between the coolant temperature and the thresholdengine load, based on the relationship such as shown in FIG. 5. Thethreshold engine load is calculated using the map, based on the detectedcoolant temperature.

FIG. 6 is a flowchart describing a control routine executed by the ECU80 in the second embodiment of the present invention. The routine ofFIG. 6 is the same as the routine of FIG. 4, with the exception that,after step S104 of FIG. 4, step S202 is executed, and after step S106step S204 is executed, and in place of step S108, steps S204 and S206are executed.

Specifically, it is determined the internal combustion engine is beingstarted from the cold condition, i.e. a cold start, at step S102 andthen information regarding the operating condition is detected (stepS104). In this case, the engine rotational speed, the acceleratoroperating amount and, in place of the coolant temperature of thecombustion chamber 24, the coolant temperature are detected inaccordance with outputs from various sensors. Next, the engine load iscalculated (step S202). The engine load is calculated based oninformation regarding the operating condition of the internal combustionengine 10 detected in step S104. Next, the requested intake air amountis calculated (step S106), and the threshold engine load is calculated(step S204). The threshold engine load is determined using a map (referto FIG. 5) stored in the ECU 80, in accordance with the coolanttemperature calculated at step S104.

Next, it is determined whether or not the current load is greater thanor equal to the threshold engine load (step S206). That is, the loadcalculated at step S202 and the load calculated at step S204 arecompared, and it is determined whether the engine load is greater thanor equal to the threshold engine load. At step S206 if it is determinedthat the engine load is greater than or equal to the threshold engineload, six-stroke operation is performed (step S110). That is, control isperformed so that the first intake stroke is performed with the intakevalve 40 at the low lift amount and the first compression stroke isperformed, and the second intake stroke is performed with the intakevalve 40 at the normal lift amount condition, the second compressionstroke, and ignition are performed, followed by the expansion stroke andthe exhaust stroke. The processing of steps S104, S202, S106, S204,S206, and S110 is repeated until it is determined that the engine loadis greater than or equal to the threshold engine load at step S206.

If, however, it is not determined that the internal combustion engine isbeing started from the cold condition at step S102, or if at step S206it is determined the engine load is greater than or equal to thethreshold engine load, four-stroke operation is set (step S112), and theprocessing is ended.

As described above, in the second embodiment the threshold engine loadfor switching from six-stroke operation to four-stroke operation is setin accordance with the coolant temperature, and the engine operation isswitched from six-stroke operation to four-stroke operation inaccordance with the set threshold engine load. Therefore, if therequested engine load is large and the internal combustion engine cannot generate an output torque in corresponding to the requested loadwith six-stroke operation, or if knocking is expected to occur becauseof abnormal combustion, it is possible to avoid six-stroke operation andperform four-stroke operation. Also, six-stroke operation is continueduntil either the first or second above-noted condition is satisfied. Forthis reason, if the intake temperature is low at the cold start of theinternal combustion engine, it is possible to reliably raise thetemperature of the intake gas and improve the combustion condition.

The second embodiment is described for the case in which the switchingload is set to the smaller of the first condition that considers therequested engine load and the second condition that considers theoccurrence of knocking. In the present invention, however, the thresholdengine load need not take into consider both of these, and may be setwith consideration given to either one of the first condition and thesecond condition.

In the second embodiment, by executing step S202 the “requested loadcalculation means” may be implemented, and by executing step S206, the“multi-stork operation termination determining means” may beimplemented.

The third embodiment of the present invention is described below, withreference made to FIG. 7. The description as follows will focus on theonly the characteristic parts of the third embodiment, and thedescriptions of parts that are the same as the first embodiment will beeither simplified or omitted. The system in the third embodiment has thesame type of configuration as the system of the first embodiment. In thesystem of the third embodiment, with the exception of predicting thetemperature in the combustion chamber 24 and switching from four-strokeoperation to six-stroke operation based on the predicted temperature,control is performed in the same manner as in the first embodiment.

Specifically, in the system of the third embodiment as well, six-strokeoperation is performed at the cold start. It is possible to predict thetemperature rise ΔT after the intake stroke in six-stroke operation,based on the air intake amount detected during the six-stroke operation.Therefore, the predicted temperature Tp in the combustion chamber 24after the second intake stroke in six-stroke operation can be expressedby the current temperature T24 in the combustion chamber and thetemperature rise ΔT in the form of Equation (1).

Predicted combustion chamber temperature Tp=Combustion chambertemperature T24+ΔT  (1)

Before starting six-stroke operation, even if the temperature in thecombustion chamber 24 is lower than the threshold engine load, there arecases in which an excessive rise occurs in the intake temperature whensix-stroke operation is actually performed. If ignition is done underthe condition, because it may cause abnormal combustion or knocking tooccur, it is preferable to avoid this condition. Therefore, in the thirdembodiment, as noted above, the temperature Tp in the combustion chamber24 after the second intake stroke in six-stroke operation is predictedand, if the predicted temperature Tp is at least the threshold engineload T0, a switch is made to four-stroke operation.

FIG. 7 is a flowchart describing a control routine executed by the ECU80 in the third embodiment. The flowchart shown in FIG. 7, with theexception of having steps S302 to S310 after the step S110 of theflowchart shown in FIG. 4, is the same as the routine shown in FIG. 4.Specifically, in the first combustion when the cold start is determinedat step S102, if at step S108 it is determined that the currenttemperature T24 in the combustion chamber 24 is lower than the thresholdcylinder temperature T0, six-stroke operation is performed (step S110),after which information regarding the operating condition, such as thetemperature T24 in the combustion chamber 24 or the intake air amountsand the like in the first and second intake strokes is again detected(step S302).

Next, the requested intake air amount is calculated (step S304). Afterthat, based on the intake air amounts in the first and second intakestrokes detected at step S302, the temperature rise ΔT is calculated(step S306). The temperature rise ΔT can be determined from a mapestablishing the relationship between the intake air amount and thetemperature rise. Next, the predicted temperature Tp after the secondintake stroke in the combustion chamber 24 is calculated (step S308).The combustion chamber predicted temperature Tp is calculated inaccordance with the above-noted Equation (1).

Next, it is determined whether the combustion chamber predictedtemperature Tp is greater than or equal to the threshold predictedcylinder temperature T0 (step S310). If it is determined that thecombustion chamber predicted temperature Tp is greater than or equal tothe threshold cylinder temperature T0, six-stroke operation is againperformed at step S110, and the processing of steps S302 to S310 isperformed. That is, as long as it is not determined that the conditioncombustion chamber predicted temperature Tp is greater than or equal tothe threshold predicted cylinder temperature T0 at step S310, six-strokeoperation is performed at step S110. If, however, it is determined thatthe combustion chamber predicted temperature Tp is greater than or equalto the threshold predicted cylinder temperature T0 at step S310,four-stroke operation is set and processing is ended.

As described above, according to the third embodiment, when the internalcombustion engine 10 is being started from the cold condition, i.e. coldstart, when six-stroke operation is performed, the combustion chambertemperature after performing six-stroke operation is predicted, and itis determined whether to switch to four-stroke operation based on thepredicted temperature. It is therefore possible to suppress an excessiverise in the temperature in the combustion chamber 24, and possible toeffectively prevent knocking due to abnormal combustion.

The third embodiment is described for the case in which the temperaturein the combustion chamber 24 is detected by the temperature sensor 26,and the predicted temperature is calculated from the detectedtemperature and the temperature rise predicted from the intake airamount. However, the method for calculating the predicted temperature Tpin the combustion chamber 24 is not limited to this method, and may be acalculation by another method. For example, the initial value of thetemperature in the combustion chamber 24 may be predicted from thecoolant temperature at the time of starting, after which the temperaturerise ΔT is calculated from the intake air amounts for the intake strokes(first intake stroke and second intake stroke) for each combustioncycle, and the temperature rise ΔT may be successively added to theinitial value of the temperature in the combustion chamber 24 to predictthe temperature in the combustion chamber 24. Also, for example, acombustion pressure sensor that detects the combustion pressure isprovided and the temperature in the combustion chamber may be predictedfrom the combustion pressure and the intake air amount. Additionally,the temperature in the combustion chamber 24 is directly detected andthe temperature at the next time may be predicted from the amount ofvariation. Alternatively, a temperature sensor is provided in thevicinity of the intake valve to directly detect the intake temperatureand the temperature in the combustion chamber 24 is predicted based onthe intake temperature.

The third embodiment was described for the case in which a switch ismade to four-stroke operation if the predicted temperature in thecombustion chamber 24 reaches or exceeds the threshold predictedcylinder temperature. The present invention, however, is not limited inthis manner, and if the predicted temperature in the combustion chamber24 is at least the threshold predicted cylinder temperature, the liftamount may be increased by a prescribed amount from the low lift amount,the low lift amount being gradually changed until it reaches the normallift amount during which time six-stroke operation is continued. Bydoing this, it is possible to suppress the torque variation to a smallamount when the switch is made to four-stroke operation. When suchcontrol is performed, the threshold predicted cylinder temperature maybe set to lower than the normal. Additionally, amount of gradual changeof the lift amount during six-stroke operation is not limited to being afixed amount of change.

In the third embodiment, by executing steps S306 and S308, the “cylinderinternal temperature predicting means” may be implemented, and byexecuting step S310 the “multi-stroke operation termination determiningmeans” may be implemented.

The fourth embodiment of the present invention is described below, withreference made to FIG. 8 and FIG. 9. The description of the fourthembodiment will focus on the only the characteristic parts of the fourthembodiment, and the descriptions of parts that are the same as the firstto third embodiments will be either simplified or omitted. The system inthe fourth embodiment has the same type of configuration as the systemof the first embodiment, with the exception that it is used as aflexible fuel vehicle (FFV). Specifically, the system of the fourthembodiment can use alcohols such as ethanol, methanol, bio-ethanol, orbio-methanol, or a mixture of these alcohols and gasoline as a fuel. Useas a fuel is possible regardless of the proportion of alcohol fuel inthe fuel that is used.

The system in the fourth embodiment performs six-stroke operation at thecold start. The control executed by the system of the fourth embodiment,with the exception of the threshold cylinder temperature, the thresholdcoolant temperature, the threshold engine load or the thresholdpredicted cylinder temperature being set in accordance with the alcoholconcentration in the fuel when determining whether to switch fromsix-stroke operation to four-stroke operation, is the same as in thefirst embodiment. FIG. 8 is a graph describing the relationship betweenthe alcohol concentration in the fuel and the threshold coolanttemperature for switching to four-stroke operation in a fourthembodiment of the present invention.

The proportion of alcohol fuel in the fuel used the system of the fourthembodiment as described above is not fixed. However, the concentrationof alcohol included in the fuel used is a factor affecting theatomization of fuel when taken into the cylinder 12. Specifically,atomization of the fuel occurs easily even at a relatively lowtemperature in the case in which the alcohol concentration is low andthe gasoline concentration is high, whereas, as the alcoholconcentration in the fuel increases, it becomes difficult for the fuelto atomize. For this reason, the temperature at which a given amount offuel can be atomized is higher, the higher is the alcohol concentration.

Therefore, particularly when the temperature in various parts of theinternal combustion engine 10 is low at the cold start, the higher thealcohol concentration is, in order to achieve stable combustion, themore it is necessary raise the temperature of the intake gas to make iteasier to atomize the fuel. For this reason, when six-stroke operationis performed for raising the intake temperature, the intake temperatureis made higher, the higher is the concentration of the alcohol fuel.That is, as shown in FIG. 8, control in which the threshold coolanttemperature for switching from six-stroke operation to four-strokeoperation is made higher, the higher is the alcohol fuel concentration,to raise the intake temperature in six-stroke operation, is continueduntil the inside of the combustion chamber 24 reaches a highertemperature.

The ECU 80 has stored map establishing the relationship, as shown inFIG. 8, between alcohol concentration in the fuel and the thresholdcoolant temperature. When making a cold start of the internal combustionengine 10, the alcohol concentration of the fuel is detected and thethreshold coolant temperature is calculated in accordance with the map,in accordance with the detected alcohol concentration. If thetemperature of the coolant in the internal combustion engine 10 reachesor exceeds the threshold coolant temperature, a switch is made fromsix-stroke operation to four-stroke operation.

FIG. 9 is a flowchart describing a control routine executed by the ECU80 in the fourth embodiment. The routine of FIG. 9, with the exceptionof execution of steps S402 to S406 in place of step S108 after step S106of FIG. 4, is the same as the routine of FIG. 4. Specifically, if it isdetermined at step S102 that the internal combustion engine is beingstarted from the cold condition, information regarding the operatingcondition is detected, the requested intake air amount is calculated(steps S104 and S106), and the alcohol concentration of the currentlyused fuel is read out (step S402). The alcohol concentration of the fuelis stored in the ECU 80. At this point, instead of reading out thealcohol concentration from the ECU, a concentration meter that detectsthe concentration of the alcohol fuel may be installed to detect thealcohol concentration.

Next, the threshold coolant temperature is calculated (step S404). Thethreshold coolant temperature is calculated as a value corresponding tothe alcohol concentration read out in step S402, in accordance with themap stored beforehand in the ECU 80. Next, it is determined whether thecoolant temperature detected at step S104 is greater than or equal tothe threshold coolant temperature (step S406) and, if it is notdetermined that the coolant temperature is greater than or equal to thethreshold coolant temperature, six-stroke operation is performed (stepS110). Six-stroke operation is repeated in the steps S104, S106, S402 toS406 and S110 until it is determined at step S406 that the coolanttemperature is greater than or equal to the threshold coolanttemperature. If it is determined that the coolant temperature is greaterthan or equal to the threshold coolant temperature at step S406, aswitch is made to four-stroke operation (step S112).

As described above, in the fourth embodiment the threshold coolanttemperature is calculated in accordance with the alcohol concentrationin the fuel. For this reason, it is possible to continue six-strokeoperation until the coolant temperature reaches a temperature, at whichthe combustion stabilizes, set in accordance with the alcoholconcentration, and possible to reliably perform warm-up to the requiredtemperature. It is also possible to accommodate combustibility inaccordance with the concentration of the alcohol fuel. When it isdifficulty to achieve stable combustion due to the high alcoholconcentration, it is possible to raise the temperature more quickly toachieve stable combustion by continuing six-stroke operation to a highertemperature. In particular when a fuel is used that includes an alcoholfuel, although there are cases in which the cold start is difficult, itis possible to raise the temperature in the combustion chamber 24 morequickly by performing six-stroke operation as in the above-describedfourth embodiment. Therefore, because the temperature in the combustionchamber becomes a temperature at which the fuel can be atomized morequickly, it is possible to improve the starting characteristics of theinternal combustion engine. In an engine using a fuel with a lowvolatility, such as in an FFV, the fourth embodiment can effectivelyimprove starting characteristics.

The fourth embodiment is described for the case of using an alcohol fuelor a fuel mixture of an alcohol fuel and gasoline. The presentinvention, however, is not limited in this manner, and may also use afuel including a so-called bio-alcohol or a light oil in place ofgasoline. In this case as well, in general if the alcohol concentrationis high, the threshold coolant temperature will be set to high. In thismanner, by experimentally setting the relationship between the thresholdcoolant temperature and the alcohol concentration for each fuelbeforehand, the forth embodiment can be applied to other alcohol fuelsas well.

The fourth embodiment is described for the case in which the thresholdcoolant temperature is set in accordance with the alcohol concentration.The fourth embodiment, however, is not limited in this manner. Forexample, the threshold coolant temperature T0 with respect to thecombustion chamber temperature T24 in the first embodiment, thethreshold engine load in the second embodiment, and the thresholdpredicted coolant temperature T0 with respect to the combustion chamberpredicted temperature Tp in the third embodiment may each be set inaccordance with the alcohol concentration. Each of these thresholds canbe made based on experimental maps in accordance with the alcoholconcentration.

Also, for example, in the fourth embodiment by executing step S302 andstep S304, the “determination value setting means” may be implemented.

The fifth embodiment of the present invention is described below, withreference made to FIG. 10. The description of the fifth embodiment willfocus on the only the characteristic parts of the fifth embodiment, andthe descriptions of parts that are the same as the first through thefourth embodiments will be either simplified or omitted. The system ofthe fifth embodiment, with the exception that the engine is a so-calledV-type engine having a plurality of cylinders, is the same as the systemof FIG. 1.

Specifically, the internal combustion engine 10 of the fifth embodimenthas two groups (hereinafter “banks”) of cylinders. In this system, ifthe requested load is large, the internal combustion engine 10 isoperated with all of the cylinders 12 operating (all-cylinderoperation). In contrast, if the requested load is small, only one bankof cylinders is operated, with cylinders belonging to the other bankbeing stopped (reduced-cylinder operation).

In the case of reduced-cylinder operation, in which only one bank isoperating, the cylinders belonging to the other bank are stopped. Inthis condition, if the requested load becomes large, transition is madefrom the reduced-cylinder operation to all-cylinder operation. That is,the bank that is stopped (auxiliary bank) is started. Then, even if thecylinders 12 on the operating bank side, which are operating duringreduced-cylinder operation, are warm up, if the reduced-cylinderoperation continued for a long period of time and when starting cold inthe reduced-cylinder operation, the cylinders 12 of the auxiliary bankthat are stopped during reduced-cylinder operation might not besufficiently warmed up. In such cases, immediately after switching fromreduced-cylinder operation to all-cylinder operation, it can beenvisioned that a raise in the temperature of the intake air in theauxiliary bank is not sufficient, and combustion in the cylinders of theauxiliary bank degrades.

In the above-noted situation, it cylinders 12 of the auxiliary bank arenot warmed up and are still cold, the system of the fifth embodimentmakes one combustion cycle of temperature six-stroke operation when theauxiliary bank is returned to operation. That is, with regard to theauxiliary bank, the low lift amount at which the pumping loss is maximumis set, and the first intake stroke and the first compression stroke areperformed, after which the lift amount is set to the normal lift amountand then the second intake stroke, the second compression stroke, theexpansion stroke, and the exhaust stroke are performed. After that, ifthe temperature T24 in the combustion chambers 24 of the auxiliary bankreaches or exceeds the threshold cylinder temperature T0, the six-strokeoperation is ended and the four-stroke operation is performed.

During this period, the current operating condition on the operatingbank that is operating during the reduced-cylinder operation ismaintained. That is, in the case of performing four-stroke operation,four-stroke operation is performed, and in the case of performingsix-stroke operation, six-stroke operation is performed, for example, inaccordance with the routine shown in FIG. 4. If it is determined thatthe six-stroke operation is to be terminated, a switch is made tofour-stroke operation. When the operation of the auxiliary bank isrestored by four-stroke operation, the internal combustion engine 10starts to operate with all of the cylinders 12 operating. When thestopped bank is returned to four-stroke operation, the ignition timingis switched to ignition timing that is set for all-cylinder operationbeforehand, and then, the timing of the intake valves 40 and the exhaustvalves 60 of each cylinder 12 is switched to the valve timing setbeforehand.

FIG. 10 is a flowchart describing a control routine executed by thesystem of the fifth embodiment. The routine of FIG. 10, is repeatedlyexecuted during operation of the internal combustion engine 10.Specifically, it is determined at step S502 whether or notreduced-cylinder operation is in progress. If it is not determined thatreduced-cylinder operation is in progress, the current operation iscontinued and processing ends.

If, however, it is determined at step S502 that reduced-cylinderoperation is in progress, next information regarding the operatingcondition is detected (step S504). Required information, for example theengine rotational speed and intake air amount, and the coolanttemperature and the like is detected based on outputs from varioussensors. Next, the current requested load is calculated (step S506). Therequested load is calculated based on the accelerator operating amount.Next, it is determined whether there is a request to transition fromreduced-cylinder operation to all-cylinder operation (step S508).Whether or not there is a request to transition from reduced-cylinderoperation to all-cylinder operation is determined, for example, based onwhether the load calculated at step S506 is higher than a prescribedload. If a request for transition to all-cylinder operation isdetermined at step S508, the current operation is continued andprocessing is ended.

If it is not determined that there is a request for transition toall-cylinder operation at step S508, however, it is determined whetherthe cold start of the auxiliary bank is carried out (step S510).Specifically, the determination is made based on whether the temperatureof the coolant in the auxiliary bank detected at step S504 is lower thana prescribed coolant temperature.

If it is determined at step S510 that the auxiliary bank is beingrestored from a cold start, the temperature T24 in the combustionchamber 24 of the cylinders 12 in the auxiliary bank is detected (stepS512). Then, it is determined whether the temperature T24 is greaterthan or equal to the threshold cylinder temperature T0 for switchingfrom the six-stroke operation to the four-stroke operation (step S514).If it is not determined that the temperature T24 is greater than orequal to the threshold cylinder temperature T0 at step S514, at stepS516 the auxiliary bank is set to six-stroke operation. That is, thefirst intake stroke is performed with the intake valve 40 set to the lowlift amount, and the first compression stroke are performed. After thefirst intake stroke and compression stroke, the second intake stroke isperformed with the intake valve 40 set to the normal lift amount andsecond compression stroke are performed, after which the expansionstroke and exhaust stroke are performed. After that, processing returnsto step S512. The six-stroke operation of steps S512, S514, and S516 isrepeated until it is determined step S514 that the temperature T24 inthe combustion chamber 24 reaches or exceeds the threshold cylindertemperature T0.

It is not determined at step S510 that the auxiliary bank of cylinders12 is being restored from the cold condition, and it is determined atstep S514 that the temperature T24 is greater than or equal to thethreshold cylinder temperature T0, at step S518 normal four-strokeoperation is executed, and all-cylinder operation is performedimmediately. After that, the processing is ended.

As described above, according to the fifth embodiment, even whenrestoring the operation of the auxiliary bank that had been stopped, byperforming six-stroke operation to raise the temperature, it is possibleto more quickly raise the temperature in the combustion chambers 24 onthe auxiliary bank, enabling stabilization of combustion.

In the system of the fifth embodiment, it is possible to start withreduced-cylinder operation even when the cold start of the internalcombustion engine 10 is carried out. In the case of performing a coldstart with reduced-cylinder operation, the routine of FIG. 4 isperformed in the same manner as in the first embodiment, and theauxiliary bank only is operated in six-stroke operation until thetemperature T24 in the combustion chambers 24 on the auxiliary bankrises to the threshold cylinder temperature T0. In this manner, evenwhen cold start of the internal combustion engine 10 is carried out, itis possible by performing six-stroke operation to raise the temperaturein the combustion chambers 24 on the auxiliary bank to more quicklystabilize combustion. Because it is possible in this manner to performreduced-cylinder operation even in the cold condition, it is possible toimprove fuel economy.

In the fifth embodiment, for example, by executing step S510 the “coldstarting determining means” may be implemented, and by executing stepS516 the “multi-stroke operation means” may be implemented.

In the case in which references are made to numbers of elements,quantities, and ranges in the like in the foregoing embodiments, unlessthe numbers are explicitly stated or clear in principle as specificnumbers, there is no restriction to the stated numbers. Additionally,the structures and methods of steps described in the embodiments, unlessexplicitly stated or clear in principle as specified structures andmethods, are not necessarily essential to the present invention.

1. A control apparatus for an internal combustion engine comprising: avariable valve driving device that changes the timing of the opening andclosing and lift amount of an intake valve disposed in an intake portthat communicates with a cylinder of the internal combustion engine; avalve timing control portion that controls the timing of the opening andclosing and lift amount of the intake valve by the variable valvedriving device; a cold start determining portion that determines whetherthe internal combustion engine is being started from a cold start; and amulti-stroke operation setting portion that sets a multi-strokeoperation, in which one combustion cycle of the internal combustionengine includes two or more intake and compression strokes, when thecold start determining portion determines that the internal combustionengine is being started from the cold start, wherein the multi-strokeoperation is formed by a first intake stroke and a first compressionstroke and a second intake stroke and a second compression stroke,followed by a combustion stroke and an exhaust stroke, wherein the valvetiming control portion controls the lift of the intake valve during thefirst intake and the compression strokes to a low lift amount, which issmaller than the normal lift amount required for intake of a requestedintake air amount, and controls the lift of the intake valve in thesecond intake and the second compression stroke to the normal liftamount.
 2. The control apparatus according to claim 1, wherein the lowlift amount is the lift at which the pumping loss during the firstintake and the first compression stroke is maximized.
 3. The controlapparatus according to claim 1, further comprising: an ignition timingcontrol portion that controls ignition timing by a spark plug disposedin the cylinder, wherein the ignition timing control portion prohibitsignition during the first intake and the first compression stroke. 4.The control apparatus according to claim 1, wherein the multi-strokeoperation setting portion sets the execution of, during one combustioncycle, a plurality of repetitions of the first intake stroke and thefirst compression stroke, followed by performing the second intakestroke and the second compression stroke.
 5. The control apparatusaccording to claim 1, further comprising: a multi-stroke operationtermination determining portion that determines whether or notmulti-stroke operation is to be terminated; and a four-stroke operationsetting portion that sets one combustion cycle of the combustion of theinternal combustion engine to four-stroke operation, which includes anintake stroke, a compression stroke, an expansion stroke, and an exhauststroke, if the multi-stroke operation termination determining portiondetermines that multi-stroke operation is to be terminated.
 6. Thecontrol apparatus according to claim 5, further comprising: atemperature detector that detects a temperature in the cylinder whereinthe multi-stroke operation termination determining portion determinesthat multi-stroke operation is to be terminated if a temperature in thecylinder is greater than or equal to a threshold cylinder temperature.7. The control apparatus according to claim 5, further comprising: acoolant temperature detector that detects a temperature of the coolantof the internal combustion engine wherein the multi-stroke operationtermination determining portion determines that multi-stroke operationis to be terminated if the temperature of the coolant is greater than orequal to a threshold coolant temperature.
 8. The control apparatusaccording to claim 5, further comprising: a requested load calculationportion that calculates a requested load on the internal combustionengine wherein the multi-stroke operation termination determiningportion determines that multi-stroke operation is to be terminated ifthe calculated requested load is greater than or equal to a thresholdengine load.
 9. The control apparatus according to claim 5, furthercomprising: a cylinder temperature predicting portion that predicts,before starting the first intake stroke and the first compressionstroke, in one combustion cycle, a temperature in a cylinder afterperforming the second intake stroke and the second compression strokewherein the multi-stroke operation termination determining portiondetermines that multi-stroke operation is to be terminated if thepredicted temperature in the cylinder is greater than or equal to athreshold predicted cylinder temperature.
 10. The control apparatusaccording to claim 6, wherein the internal combustion engine uses a fuelincluding alcohol as a fuel, and the control apparatus further includesa determining value setting portion that sets any one of the thresholdcylinder temperature, the threshold coolant temperature, the thresholdengine load, and the threshold predicted cylinder temperature inaccordance with the concentration of alcohol in the fuel.
 11. Thecontrol apparatus according to claim 1, wherein the internal combustionengine has a first cylinder group and a second cylinder group, andwherein the control apparatus operates only cylinders belonging to thefirst cylinder group and includes a reduced-cylinder operation settingportion that sets cylinders belonging to the second cylinder group toreduced cylinder operation, in which the cylinders are stopped, and anall-cylinder operation setting portion that sets all cylinders belongingto the first cylinder group and cylinders belonging to the secondcylinder group to all-cylinder operation, in which all cylinders areoperated, wherein the cold start determining portion determines whetherrestoration of operation of the cylinders belonging to the second groupof cylinders is a cold start when the engine transitions fromreduced-cylinder operation to all-cylinder operation, and themulti-stroke operation setting portion sets the operation of cylindersbelonging to the second cylinder group to multi-stroke operation whenthe cold start determining portion determines that restoration ofoperation of the cylinders belonging to the second cylinder group ofcylinders is the cold start.
 12. The control apparatus according toclaim 1, wherein the variable valve driving device has an intake camthat drives the opening and closing of the intake valve and anelectrical motor rotationally driving the intake cam, wherein the valvetiming control portion controls the valve timing by controlling therotational drive of the intake cam using the electrical motor.
 13. Amethod for controlling an internal combustion engine comprising:determining whether the internal combustion engine is being started froma cold start; executing multi-stroke operation, which includes, in onecombustion cycle in the internal combustion engine, two or more intakeand compression strokes, formed by a first intake stroke and a firstcompression stroke and a second intake stroke and compression stroke,followed by a combustion stroke and an exhaust stroke; and controlling alift of an intake valve during the first intake stroke and the firstcompression stroke to a low lift amount, which is smaller than thenormal lift amount required for intake of a requested intake air amount,and controlling the lift of the intake valve in the second intake strokeand the second compression stroke to the normal lift amount.
 14. Thecontrol apparatus according to claim 7, wherein the internal combustionengine uses a fuel including alcohol as a fuel, and the controlapparatus further includes a determining value setting portion that setsany one of the threshold cylinder temperature, the threshold coolanttemperature, the threshold engine load, and the threshold predictedcylinder temperature in accordance with the concentration of alcohol inthe fuel.
 15. The control apparatus according to claim 8, wherein theinternal combustion engine uses a fuel including alcohol as a fuel, andthe control apparatus further includes a determining value settingportion that sets any one of the threshold cylinder temperature, thethreshold coolant temperature, the threshold engine load, and thethreshold predicted cylinder temperature in accordance with theconcentration of alcohol in the fuel.
 16. The control apparatusaccording to claim 9, wherein the internal combustion engine uses a fuelincluding alcohol as a fuel, and the control apparatus further includesa determining value setting portion that sets any one of the thresholdcylinder temperature, the threshold coolant temperature, the thresholdengine load, and the threshold predicted cylinder temperature inaccordance with the concentration of alcohol in the fuel.